Mary can mow a lawn in four hours and Tom can mow the lawn in 5 hours. If Tom works for 2 hours alone, what fractional part of the lawn remains to be mowed?
Solution: If Tom can mow a lawn in $5$ hours, then in one hour, he can mow $1/5$ of the lawn. Since he mows for $2$ hours, he finished mowing $2 \times \frac{1}{5} = \frac{2}{5}$ of the lawn. This leaves $1 - \frac{2}{5} = \boxed{\frac{3}{5}}$ of the lawn left to be mowed.